The morphology
of lava describes its surface form or texture. Deep-water submarine lavas
can be categorized as sheet flows or pillow flows. Sheet flows are always
broad, relatively flat, and fill in low areas in the landscape. This is
because they are very fluid when erupted, as reflected in their various
morphologies: "ropy", "lineated", "lobate",
or "jumbled"(if their surface becomes disrupted). Pillow flows,
on the other hand, are comprised of individual bulbous lobes of lava that
are piled one upon the next, and tend to form large haystack-shaped mounds
or ridges. Different parts of a single flow can have different lava morphologies.

When it
is first erupted molten lava is very mobile. But in the frigid deep ocean
its surface is cooled very quickly and it starts to form a solid crust.
Pillow lavas usually solidify completely in place. However, beneath the
frozen carapace of sheet flows, molten lava often continues to move freely
for hours after eruption before finally solidifying. Because of this,
sheet flows tend to be at least partially hollow, where the molten interior
of the flow drained back into the eruptive fissure or continued downslope
before freezing. Where they are hollow, flow roofs often collapse under
their own weight, sometimes revealing lava pillars locally supporting
a portion of the uncollapsed carapace.